This invention relates generally to battery charging circuitry and methods and, more particularly, relates to methods and apparatus for measuring a battery charging current, as well as a battery discharging current.
A battery charging circuit of most interest to these teachings is one used with a wireless terminal, also referred to as mobile station or as a personal communicator. During a charging operation a relatively high current is required to be measured and monitored, typically in the range of several hundred milliamps (mA) or even more. Referring to FIG. 1, conventional practice places a series resistance (Rmeas) between a source of charging current, shown as a charger 1, and the associated charger switch (Msw) 2. Msw is coupled to the battery 3 to be recharged. The battery charging current (Ich) flows through Rmeas, and the resulting voltage drop (Vmeas) across Rmeas is sensed for controlling the charging cycle. For example, an analog-to-digital converter (ADC) 4 may be used to convert Vmeas to an a digital representation, which in turn may be used to modulate the pulse width of a signal output from a pulse width modulator (PWM) 5. Vmeas may also be employed as a measurement of the battery voltage. The output of the PWM 5 can be used directly, or it can be further modified by a charging controller 6, to provide a switching signal (Vcntrl) to Msw. In this way the conduction through Msw is varied so that as the battery 3 reaches full charge the on-time of Msw can be gradually reduced until finally Msw is supplying only a maintenance (trickle) charge to the battery 3. In other embodiments the feedback loop from Rmeas to Vcntrl could be implemented in an entirely analog fashion, or in a mixed analog/digital fashion.
One significant drawback to the use and operation of this type of conventional charging circuit is that Rmeas is required to be a low ohmic value, high precision resistor. Due to the significant current flow through Rmeas it must also be large physically in order to dissipate the resulting heat. The use of a physically large resistor implies that a separate, discrete component be used, as opposed to an integrated component, which increases the cost as well as the complexity of the manufacturing and testing operations. Furthermore, Rmeas must be carefully located so as not to excessively heat adjacent circuit components. In addition, because of the low ohmic value of Rmeas the resulting voltage drop Vmeas is also small, which can require the use of high resolution ADC 4 to obtain an accurate measurement of Ich.
It is a first object and advantage of this invention to provide an improved battery energy management circuit.
It is another object and advantage of this invention to provide an improved battery charging circuit for use in a wireless terminal that overcomes the foregoing and other problems.
It is a further object and advantage of this invention to provide an improved battery discharging circuit for use in making battery capacity and other types of measurements.
The foregoing and other problems are overcome and the foregoing objects and advantages are realized by methods and apparatus in accordance with embodiments of this invention.
Disclosed herein is a method for charging a battery and circuitry for performing the method. The method includes steps of: (a) generating a charging current (Ich) for a battery; (b) generating a replica current (Irep) of Ich, where Irep=Ich/N, where N greater than 1; (c) measuring a voltage drop induced by Irep across a measurement resistance; and (d) using the measured voltage drop for controlling a magnitude of Ich. Preferably N is greater than about 10, more preferably N is greater than about 100, and in the most preferred embodiment N is in a range of about 100 to about 1000. The step of generating the charging current (Ich) includes a step of operating a first device having an input node coupled to a source of charging current, the step of generating the replica current (Irep) includes a step of operating a second device having an input node coupled to the source of charging current; wherein the first device and the second device are both driven with the same control signal. The control signal may be a pulse width modulated signal having a pulse width that is controlled as a function of the measured voltage drop across the measurement resistance. The control signal may instead be a DC voltage with an adjustable voltage value that is controlled as a function of the measured voltage drop across the measurement resistance. In the preferred embodiment the step of generating the replica current (Irep) includes a step of operating a servo loop to force a potential appearing at an output node of the second device to equal a potential appearing at an output node of the first device.
A battery charging circuit in accordance with these teachings includes a first device driven by a control signal that controls the on and off times and/or the conduction through of the first device and, hence, the amount of charging current Ich supplied to the battery being recharged. The first device has an input node coupled to a source of charging current and an output node for coupling the charging current Ich to the battery to be recharged. The battery charging circuit further includes a second device that is driven by the control signal and that has an input node coupled to the source of charging current and an output node coupled to a measurement resistance. The voltage drop across the measurement resistance due to a current flow Irep through the measurement resistance is sensed for controlling the conduction of the first device.
Preferably Irep is equal to Ich/N, where N is a scaling factor that is greater than unity.
The circuit further includes a difference amplifier having a first input coupled to the output node of the first device, a second input coupled to the output node of the second device, and an output coupled to a control terminal of a transistor coupled in series with the measurement resistance. The difference amplifier operates the transistor for forcing a voltage potential appearing at the output node of the second device to be equal to a voltage potential appearing at the output node of the first device.
The measurement resistance can be coupled in series between the output node of the second device and an input node of the transistor, or the transistor can be is coupled in series between the output node of the second device and the measurement resistance.
The disclosed circuitry and method may be extended for providing a battery discharge measurement circuit for enabling a battery capacity test to be performed.